Strained-channel transistor structure with lattice-mismatched zone

ABSTRACT

A strained-channel transistor structure with lattice-mismatched zone and fabrication method thereof. The transistor structure includes a substrate having a strained channel region, comprising a first semiconductor material with a first natural lattice constant, in a surface, a gate dielectric layer overlying the strained channel region, a gate electrode overlying the gate dielectric layer, and a source region and drain region oppositely adjacent to the strained channel region, with one or both of the source region and drain region comprising a lattice-mismatched zone comprising a second semiconductor material with a second natural lattice constant different from the first natural lattice constant.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device withlattice-mismatched zone and fabrication method thereof, and morespecifically to a strained-channel transistor structure and fabricationmethod thereof.

2. Description of the Related Art

Size reduction of the metal-oxide-semiconductor field-effect transistor(MOSFET), including reduction of gate length and gate oxide thickness,has enabled a continuous improvement in speed performance, density, andcost per unit function of integrated circuits during the past fewdecades.

In order to further enhance performance of the transistor, strain may beintroduced in the transistor channel to improve carrier mobility toenhance performance of the transistor in addition to device scaling.There are several existing approaches to introducing strain in a channelregion of the transistor.

In one conventional approach, as described in a paper titled “NMOS andPMOS transistors fabricated in strained silicon/relaxedsilicon-germanium structures”, disclosed by J. Welser et al., publishedat the December 1992 International Electron Devices Meeting held in SanFrancisco, Calif., pp. 1000-1002, a relaxed silicon germanium (SiGe)buffer layer 110 is provided beneath channel region 126, as shown inFIG. 1A. In FIG. 1B and FIG. 1C, a simple model of different latticeconstants is used to show the intersection between relaxed SiGe layer114 of buffer layer 110 and strained-Si layer 130. In FIG. 1B, model 135shows the natural lattice constant of Si, smaller than that of SiGeshown by model 115. In FIG. 1C, when a thin layer of epitaxial Si (model135) is grown on relaxed SiGe layer 114 (model 115), unit cell 136 of Sishown in model 135 is stretched laterally so as to be under a biaxialtensile strain. The thin layer of epitaxial Si becomes strain-Si layer130 shown in FIG. 1A. In FIG. 1A, a transistor formed on the epitaxialstrained-Si layer 130 is therefore with a channel region 126 under thebiaxial tensile strain. In this approach, relaxed SiGe layer 114 is astressor that introduces strain in channel region 126. The stressor, inthis case, is placed below channel region 126. Significant mobilityenhancement has been reported for both electrons and holes in bulktransistors using a silicon channel under biaxial tensile strain. In theaforementioned approach, the epitaxial silicon layer 130 is strainedbefore forming the transistor. Therefore, there are some concerns aboutpossible strain relaxation in the subsequent high temperature CMOSprocesses. Further, the approach is very expensive since SiGe bufferlayer 110 with thickness in the order of micrometers has to be grown.Numerous dislocations exist in relaxed SiGe layer 114, some of whichpropagate to strained-Si layer 130, resulting in high defect density,thereby negatively affecting transistor performance.

In another approach, strain in a channel region is introduced after atransistor is formed. In this approach, a high stress film 220 is formedover a completed transistor structure 250, as shown in FIG. 2. Highstress film 220, being a stressor, exerts significant influence onchannel region 206, modifying silicon lattice spacing in channel region206, and thus introducing strain in channel region 206. In this case,the stressor is placed above completed transistor structure 250,described in detail in a paper disclosed by A. Shimizu et al., entitled“Local mechanical stress control (LMC): a new technique for CMOSperformance enhancement”, published in pp. 433-436 of the Digest ofTechnical Papers of the 2001 International Electron Device Meeting. Thestrain contributed by high stress film 220 is believed to be uniaxial innature with a direction parallel to a source-to-drain direction.However, uniaxial tensile strain in the source-to-drain degrades holemobility while uniaxial compressive strain degrades electron mobility.Ion implantation of germanium can be used to selectively relax thestrain so that the hole or electron mobility is not degraded, but can bedifficult to implement due to the close proximity of the N and P-channeltransistors.

SUMMARY OF THE INVENTION

Thus, the main object of the present invention is to provide atransistor structure with a strained channel region.

Another object of the present invention is to provide a strained-channeltransistor structure where portions of one or both of a source regionand drain region adjacent to a strained channel region are latticemismatched with respect to the channel region.

Another object of the present invention is to provide a fabricationmethod of a strained-channel transistor structure.

In order to achieve the above objects, the present invention provides astrained-channel transistor structure comprising a strained channelregion, a gate dielectric layer, a gate electrode, and a source regionand drain region. The substrate comprises a first semiconductor materialwith a first natural lattice constant. The gate dielectric layer is onthe strained channel region. The gate electrode is on the gatedielectric layer. The source region and drain region are oppositelyadjacent to the strained channel region. One or both of the sourceregion and drain region comprise a lattice-mismatched zone comprising asecond semiconductor material with a second natural lattice constantdifferent from the first natural lattice constant.

The present invention further provides a fabrication method of astrained-channel transistor structure. First, a substrate having achannel region, comprising a first semiconductor material with a firstnatural lattice constant, in a surface, a gate dielectric on the channelregion, and a gate electrode on the gate dielectric layer is provided.Then, a first source region and drain region are formed oppositelyadjacent to the channel region. Next, a spacer is formed on a sidewallof the gate electrode, covering a part of the surface of the substrate.Next, the surface of the substrate not covered by the spacer and gateelectrode is recessed. Further, the recess is filled with a secondsemiconductor material with a second natural lattice constant differentfrom the first natural lattice constant as a lattice-mismatched zonestraining the channel region. Finally, a second source region is formedadjacent to the first source region and a second drain region is formedadjacent to the first drain region, one or both of the second sourceregion and second drain region comprising the lattice-mismatched zone.

The present invention further provides a fabrication method of astrained-channel transistor structure. First, a substrate having achannel region, comprising a first semiconductor material with a firstnatural lattice constant, in a surface, a gate dielectric on the channelregion, a gate electrode on the gate dielectric layer, a source regionand drain region oppositely adjacent to the channel region, and a spaceron a sidewall of the gate electrode, covering a part of the surface ofthe substrate, is provided. Then, one or both of the source region anddrain region is recessed. Finally, the recess is filled with a secondsemiconductor material with a second natural lattice constant differentfrom the first natural lattice constant as a lattice-mismatched zonestraining the channel region.

The present invention further provides a fabrication method of astrained-channel transistor structure. First, a substrate having achannel region, comprising a first semiconductor material with a firstnatural lattice constant, in a surface, a gate dielectric on the channelregion, a gate electrode on the gate dielectric layer, and a spacer on asidewall of the gate electrode, covering a part of the surface of thesubstrate, is provided. Then, the surface of the substrate not coveredby the spacer and gate dielectric layer is recessed to form a recess.Finally, a source region and drain region are formed oppositely adjacentto the channel region, one or both of the source region and drain regioncomprising the lattice-mismatched zone.

The present invention further provides a fabrication method of astrained-channel transistor structure. First, a substrate having achannel region, comprising a semiconductor material, in a surface, agate dielectric on the channel region, a gate electrode on the gatedielectric layer, a source region and drain region oppositely adjacentto the channel region, and a spacer on a sidewall of the gate electrode,covering a part of the surface of the substrate, is provided. Finally,an element with an atomic size different from that of the semiconductormaterial is implanted to form a lattice-mismatched zone in one or bothof the source region and drain region straining the channel region.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more fully understood by reading thesubsequent detailed description in conjunction with the examples withreferences made to the accompanying drawings, wherein:

FIGS. 1A through 1C are cross sections illustrating a conventionalstrained silicon transistor with a relaxed SiGe layer as a stressor toinduce strain in the top epitaxial strained silicon layer.

FIG. 2 is a cross section illustrating another conventional strainedsilicon transistor introducing strain in the channel using a high stressfilm as a stressor.

FIG. 3A and FIG. 3B are cross sections illustrating strained-channeltransistor structures in accordance with the first embodiment of thepresent invention.

FIGS. 4A through 4D are cross sections illustrating strained-channeltransistor structures in accordance with the second embodiment of thepresent invention.

FIG. 5 is a flowchart illustrating, in detail, the fabrication method ofa strained-channel transistor structure in accordance with the thirdembodiment of the present invention.

FIGS. 6A through 6F are cross sections illustrating steps of thefabrication method of a strained-channel transistor structure inaccordance with the third embodiment of the present invention.

FIG. 7 is a flowchart illustrating, in detail, the fabrication method ofa strained-channel transistor structure in accordance with the fourthembodiment of the present invention.

FIGS. 8A through 8D are cross sections illustrating steps of thefabrication method of a strained-channel transistor structure inaccordance with the fourth embodiment of the present invention.

FIG. 9 is a flowchart illustrating, in detail, the fabrication method ofa strained-channel transistor structure in accordance with the fifthembodiment of the present invention.

FIGS. 10A through 10G are cross sections illustrating steps of thefabrication method of a strained-channel transistor structure inaccordance with the fifth embodiment of the present invention.

FIGS. 11A through 11D are cross sections illustrating steps of thefabrication method of a strained-channel transistor structure inaccordance with the sixth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following embodiments are intended to illustrate the invention morefully without limiting their scope, since numerous modifications andvariations will be apparent to those skilled in this art.

First Embodiment

In the first embodiment of the present invention, two kinds of stressmodes exerted on the strained-channel region according to the presentinvention are discussed.

In FIG. 3A, a cross section of a strained-channel transistor structure 3a in accordance with the first embodiment of the present invention isshown. Substrate 300 a, having a strained channel region 306 a in asurface, comprises a semiconductor material. Gate dielectric layer 312a, preferably with a thickness between about 3 and 100 Å, is on thestrained channel region 306 a. Gate electrode 314 a is on the gatedielectric layer 312 a. Spacer 316 a is on a sidewall of gate electrode314 a and covers a part of the surface of substrate 300 a. Drain region307 a, comprising drain extension region 301 a and deeper drain region302 a, and source region 308 a, comprising source extension region 303 aand deeper source region 304 a, are oppositely adjacent to strainedchannel region 306 a. Lattice-mismatched zone 305 a, comprising anothersemiconductor material with a natural lattice constant different fromthat of substrate 300 a, may be in one or both of the deeper drainregion 302 a and deeper source region 304 a. Strained channel region 306a is therefore strained by the different lattice constants of strainedchannel region 306 a and lattice-mismatched zone 305 a.

In strained-channel transistor structure 3 a in accordance with thefirst embodiment of the present invention, substrate 300 a preferablycomprises silicon, with a natural lattice constant of approximately5.431 Å, and lattice-mismatched zone 305 a preferably comprises an alloysemiconductor material such as a silicon-germanium alloy, with a naturallattice constant between about 5.431 Å to 5.657 Å depending onconcentration of germanium in the silicon-germanium alloy, larger thanthat of substrate 300 a. Molar fraction of germanium in thesilicon-germanium alloy of lattice-mismatched zone 305 a in accordancewith the first embodiment of the present invention is preferably betweenabout 0.1 and 0.9. Therefore, lattice-mismatched zone 305 a as astressor exerts a compressive stress C₁ in a source-to-drain directionand tensile stress T₁ in a vertical direction on the strained channelregion 306 a, resulting in strained channel region 306 a under acompressive strain in the source-to-drain direction and tensile strainin the vertical direction. Hole mobility in the strained channel region306 a is significantly enhanced, enhancing drive current whenstrained-channel transistor structure 3 a in accordance with the firstembodiment of the present invention is a P-channel transistor structure.

In FIG. 3B, a cross section of a strained-channel transistor structure 3b in accordance with the first embodiment of the present invention isshown. Substrate 300 b, having a strained channel region 306 b in asurface, comprises a semiconductor material. Gate dielectric layer 312b, preferably with a thickness between about 3 and 100 Å, is on thestrained channel region 306 b. Gate electrode 314 b is on the gatedielectric layer 312 b. Spacer 316 b is on a sidewall of gate electrode314 b and covers a part of the surface of substrate 300 b. Drain region307 b, comprising drain extension region 301 b and deeper drain region302 b, and source region 308 b, comprising source extension region 303 band deeper source region 304 b, are oppositely adjacent to strainedchannel region 306 b. Lattice-mismatched zone 305 b, comprising anothersemiconductor material with a natural lattice constant different fromthat of substrate 300 b, may be in one or both of the deeper drainregion 302 b and deeper source region 304 b. Strained channel region 306b is therefore strained by the difference lattice constants of strainedchannel region 306 b and lattice-mismatched zone 305 b.

In strained-channel transistor structure 3 b in accordance with thefirst embodiment of the present invention, substrate 300 b preferablycomprises silicon and lattice-mismatched zone 305 b preferably comprisesan alloy semiconductor material such as a silicon-carbon alloy, with anatural lattice constant smaller than that of substrate 300 b. Molarfraction of carbon in the silicon-carbon alloy of lattice-mismatchedzone 305 b in accordance with the first embodiment of the presentinvention is preferably between about 0.01 and 0.04. Therefore,lattice-mismatched zone 305 b as a stressor exerts a tensile stress T2in a source-to-drain direction and compressive stress C2 in a verticaldirection on the strained channel region 306 b, resulting in strainedchannel region 306 b under a tensile strain in the source-to-draindirection and compressive strain in the vertical direction. Electronmobility in the strained channel region 306 b is significantly enhanced,enhancing drive current when strained-channel transistor structure 3 bin accordance with the first embodiment of the present invention is anN-channel transistor structure. Furthermore, lattice-mismatched zone 305b may further comprise germanium as a silicon-germanium-carbon alloy, inwhich molar fraction of carbon is more than a tenth of that ofgermanium.

Further, compressive strain and tensile strain of strained channelregion 306 a in FIG. 3A and strained channel region 306 b in FIG. 3B areabout 0.1% to 4%, preferably about 1% to 4%. Both lattice-mismatchedzone 305 a in FIG. 3A and lattice-mismatched zone 305 b in FIG. 3B areabout 10 Å and 1000 Å thick. Compressive strain and tensile strain ofstrained channel region 306 a in FIG. 3A and strained channel region 306b in FIG. 3B are dependent on lattice constants of lattice-mismatchedzones 306 a and 306 b, thicknesses of lattice-mismatched zones 306 a and306 b, and arrangement of lattice-mismatched zone 306 a in drain region307 a and/or source region 308 a, and lattice-mismatched zone 306 b indrain region 307 b and/or source region 308 b.

Second Embodiment

In the second embodiment of the present invention, different kinds ofarrangements of lattice-mismatched zone in drain region and/or sourceregion according to the present invention are discussed. Instrained-channel transistor structures 4 a through 4d in FIGS. 4Athrough 4D, relationships among substrate 400, drain extension region401, deeper drain region 402, drain region 407, source extension region403, deeper source region 404, source region 408, lattice-mismatchedzone 405 a/405 b/ 405 c/ 405 d, strained channel region 406 a/406 b/406c/ 406 d, gate dielectric layer 412, gate electrode 414, and spacer 416,the same as those described in the first embodiment of the presentinvention, are omitted.

In FIG. 4A, lattice-mismatched zone 405 a is arranged near the surfaceof drain region 407 and/or source region 408, and does not extend intodrain extension region 401 and/or source extension region 403. In FIG.4B, lattice-mismatched zone 405 b may protrude from the surface of drainregion 407 and/or source region 408, forming a raised drain region 407 band raised source region 408 b. In FIG. 4C, lattice-mismatched zone 405c is arranged near the surface of drain region 407 and/or source region408, and may further extend into drain extension region 401 and/orsource extension region 403. In FIG. 4D, lattice-mismatched zone 405 dis arranged more deeply from the surface of drain region 407 and/orsource region 408, and may further extend into strained channel region406 d beneath drain extension region 401 and/or source extension region403. Note that the arrangements of lattice-mismatched zone in drainregion and/or source region in accordance with the first embodiment ofthe present invention are not meant to be restrictive. Those skilled inthe art may further adjust the arrangement of lattice-mismatched zoneaccording to the present invention when required.

In FIGS. 4A through 4D, a conductive layer 420 such as silicon, metal,metal silicide, or combinations thereof is optionally formed on thesurface of drain region 407 and/or source region 408 of strained-channeltransistor structures 4 a, 4 c, and 4 d, and raised drain region 407 band/or raised source region 408 b of strained-channel transistorstructure 4 b.

Moreover, enhancement of either electron or hole mobility instrained-channel structure 4 c in FIG. 4C is further improved becauselattice-mismatched zone 405 c is closer to strained channel region 406c, resulting in lattice-mismatched zone 405 c exerting more strain onstrained channel region 406 c.

Third Embodiment

In the third embodiment of the present invention, a fabrication methodof a strained-channel transistor structure according to the presentinvention is described. A flowchart of the fabrication method inaccordance with the third embodiment of the present invention is shownin FIG. 5. The subsequent descriptions of the fabrication method inaccordance with the third embodiment of the present invention follow thesteps in FIG. 5.

In FIG. 6A, a semiconductor substrate such as a silicon substrate 500 isprovided. Silicon substrate 500 comprises a previously formed pluralityof isolation regions (not shown) and previously defined plurality ofdevice regions (not shown). For example, the isolation regions may beshallow trench isolation regions. FIGS. 6A through 6F provide across-sectional view of one single device region for easy description.Silicon substrate 500 comprises, a channel region 506 in an activesurface. Silicon substrate 500 is N-type doped when strained-channeltransistor structure 5 in FIG. 6E is a P-channel transistor structure,or P-type doped when strained-channel transistor structure 5 in FIG. 6Eis an N-channel transistor structure.

In FIG. 6B, a gate dielectric layer 512 is formed on the channel region506, and then a gate electrode 514 is formed on the gate dielectriclayer 512. Gate dielectric layer 512 is formed by thermal oxidation,thermal oxidation followed by nitridation, chemical vapor deposition,physical vapor deposition such as sputtering, or other known techniquesto form a gate dielectric layer. Gate dielectric layer 512 can besilicon dioxide, silicon oxynitride, or combinations thereof with athickness between about 3 Å to 100 Å, preferably approximately 10 Å orless. Gate dielectric layer 512 may be a high permittivity (high-k)material such as aluminum oxide (Al₂O₃), hafnium oxide (HfO₂), zirconiumoxide (ZrO₂), hafnium oxynitride (HfON), hafnium silicate (HfSiO₄),zirconium silicate (ZrSiO₄), lanthanum oxide (La₂O₃), or combinationsthereof with an equivalent oxide thickness between about 3 Å to 100 Å.Gate electrode 514 is polycrystalline-silicon (poly-Si),poly-crystalline silicon-germanium (poly-SiGe), refractory metal such asmolybdenum or tungsten, compounds such as titanium nitride, combinationsthereof or other conductive material(s). Implants known as workfunctionimplants may be introduced in gate electrode 514 to alter theworkfunction thereof. Gate electrode 514 is formed by depositing a gateelectrode material layer (not shown) over substrate 500, depositing agate mask (not shown) over the gate electrode material layer, patterningthe gate mask to define gate electrode 514, etching the gate electrodematerial layer to form gate electrode 514, and removing the gate mask.Gate electrode 514 is electrically isolated from channel region 506 bygate dielectric layer 512. Gate dielectric layer 512 is preferablysilicon oxynitride and gate electrode 514 is preferably poly-siliconetched using chlorine and bromine chemistry with a high etch selectivitywith respect to gate dielectric later 514 in accordance with the thirdembodiment of the present invention.

In FIG. 6C, a drain extension region 501 and source extension region 503are formed oppositely adjacent to channel region 506 in the activesurface of substrate 500, and spacer 516 is formed on a sidewall of gateelectrode 514, covering a part of drain extension region 501 and sourceextension region 503. The drain extension region 501 and sourceextension region 503 are formed by ion implantation, plasma immersionion implantation (PIII), or other known techniques. Spacer 516 ispreferably formed by depositing a spacer material layer (not shown) suchas silicon nitride or silicon dioxide, and selectively etching thespacer material layer. Spacer 516 is silicon nitride in accordance withthe third embodiment of the present invention.

In FIG. 6D, a part or all of the active surface of substrate 500 notcovered by gate dielectric layer 512 and spacer 516 is recessed to format least one recess 509 with a depth d by plasma etching using chlorineand bromine chemistry. The depth d of recess 509 is about 50 Å to 1000Å. An optional anneal may be performed to facilitate silicon migrationto repair etch damage caused by etching recess 509 to smooth the surfaceof recess 509 for a subsequent epitaxy process.

In FIG. 6E, recess 509 is filled with a semiconductor material such assilicon-germanium alloy or silicon-carbon alloy to form alattice-mismatched zone 505, and then a deeper drain region 502 isformed adjacent to the drain extension region 501 and a deeper sourceregion 504 is formed adjacent to the source extension region 503. Deeperdrain region 502 combines with drain extension region 501 to form adrain region 507, and deeper source region 504 combines with sourceextension region 503 to form a source region 508. One or both of thedrain region 507 and source region 508 comprise the lattice-mismatchedzone 505. Channel region 506 is strained to be a strained channel region506′ when lattice-mismatched zone 505 is formed. Thus, strained-channeltransistor structure 5 in accordance with the third embodiment of thepresent invention is basically formed. Lattice-mismatched zone 505 isformed using an epitaxy process such as chemical vapor deposition,ultra-high vacuum chemical vapor deposition, or molecular beam epitaxy.Lattice-mismatched zone 505 comprises silicon-germanium alloy wheremolar fraction of germanium is about 0.1 to 0.9 when strained-channeltransistor structure S is a P-channel transistor structure. Whenstrained-channel transistor structure 5 is an N-channel transistorstructure, lattice-mismatched zone 505 comprises silicon-carbon alloywhere molar fraction of carbon is between about 0.01 and 0.04, and mayfurther comprise germanium to be silicon-carbon-germanium alloy wheremolar fraction of germanium is less than ten times that of carbon. Asilicon cap 522 is optionally formed on lattice-mismatched zone 505using an epitaxy process such as chemical vapor deposition, ultra-highvacuum chemical vapor deposition, or molecular beam epitaxy.Lattice-mismatched zone 505 and optional silicon cap 522 may be in-situdoped or undoped during the epitaxy process. When lattice-mismatchedzone 505 and optional silicon cap 522 are undoped during the epitaxyprocess, they will be subsequently doped, with dopants activated using arapid thermal annealing process. Deeper drain region 502 and deepersource region 504 are formed using ion implantation, Pill, gas or solidsource diffusion, or other known techniques. An annealing step isfurther added to recover implant damage or amorphization duringformation of lattice-mismatched zone 505, silicon cap 522, deeper drainregion 502, and deeper source region 504.

In FIG. 6F, a conductive layer 520 is optionally formed onlattice-mismatched zone 505 and/or drain region 507/source region 508 toreduce resistances of drain region 507 and source region 508. Conductivelayer 520 is formed using self-aligned silicide, or other metaldeposition process. Passivation layers and device contacts aresubsequently formed so as to finish a device comprising strained-channeltransistor structure 5 in accordance with the third embodiment of thepresent invention.

Fourth Embodiment

In the fourth embodiment of the present invention, a fabrication methodof a strained-channel transistor structure where lattice-mismatched zonedoes not extend into drain extension region and/or source extensionregion according to the present invention is described. A flowchart ofthe fabrication method in accordance with the fourth embodiment of thepresent invention is shown in FIG. 7. The subsequent descriptions of thefabrication method in accordance with the fourth embodiment of thepresent invention follow the steps in FIG. 7.

In FIG. 8A, a semiconductor substrate such as a silicon substrate 600 isprovided. Silicon substrate 600 comprises a previously formed pluralityof isolation regions (not shown) and previously defined plurality ofdevice regions (not shown). For example, the isolation regions may beshallow trench isolation regions. FIGS. 8A through 8D provide across-sectional view of one single device region for easy description.Silicon substrate 600 comprises a conventional transistor structurecomprising a channel region 606 in an active surface of substrate 600, agate dielectric layer 612 on channel region 606, a gate electrode 614 ongate dielectric layer 612, a source region 608 and drain region 607oppositely adjacent to channel region 606, and a spacer 616 on asidewall of gate electrode 614, covering a part of the active surface ofsubstrate 600. Drain region 607 comprises a drain extension region 601and a deeper drain region 602, and source region 608 comprises a sourceextension region 603 and a deeper source region 604. Silicon substrate600 is N-type doped when strained-channel transistor structure 6 in FIG.8C is a P-channel transistor structure, or P-type doped whenstrained-channel transistor structure 6 in FIG. 8C is an N-channeltransistor structure.

In FIG. 8B, a part or all of the active surface of substrate 600 notcovered by gate dielectric layer 612 and spacer 616 is recessed to format least one recess 609 with a depth d by plasma etching using chlorineand bromine chemistry. The depth d of recess 609 is about 50 Å to 1000Å. An optional anneal may be performed to facilitate silicon migrationto repair etch damage caused by etching recess 609 to smooth the surfaceof recess 609 for a subsequent epitaxy process.

In FIG. 8C, recess 609 is filled with a semiconductor material such assilicon-germanium alloy or silicon-carbon alloy to form alattice-mismatched zone 605. One or both of the drain region 607 andsource region 608 comprise the lattice-mismatched zone 605. Channelregion 606 is strained to be a strained channel region 606′ whenlattice-mismatched zone 605 is formed. Thus, strained-channel transistorstructure 6 in accordance with the fourth embodiment of the presentinvention is basically formed. Lattice-mismatched zone 605 may protrudefrom the surface of drain region 607 and/or source region 608, forming araised drain region and raised source region. Lattice-mismatched zone605 is formed using an epitaxy process such as chemical vapordeposition, ultra-high vacuum chemical vapor deposition, or molecularbeam epitaxy. Lattice-mismatched zone 605 comprises silicon-germaniumalloy where molar fraction of germanium is about 0.1 to 0.9 whenstrained-channel transistor structure 6 is a P-channel transistorstructure. When strained-channel transistor structure 6 is an N-channeltransistor structure, lattice-mismatched zone 605 comprisessilicon-carbon alloy where molar fraction of carbon is between about0.01 and 0.04, and may further comprise germanium to besilicon-carbon-germanium alloy where molar fraction of germanium is lessthan ten times that of carbon. A silicon cap 622 is optionally formed onlattice-mismatched zone 605 using an epitaxy process such as chemicalvapor deposition, ultra-high vacuum chemical vapor deposition, ormolecular beam epitaxy. Lattice-mismatched zone 605 and optional siliconcap 622 are in-situ doped during the epitaxy process.

In FIG. 8D, a conductive layer 620 is optionally formed onlattice-mismatched zone 605 and/or drain region 607/source region 608 toreduce resistances of drain region 607 and source region 608. Conductivelayer 620 is formed using self-aligned suicide, or other metaldeposition process. Passivation layers and device contacts aresubsequently formed so as to finish a device comprising strained-channeltransistor structure 6 in accordance with the fourth embodiment of thepresent invention.

Fifth Embodiment

In the fifth embodiment of the present invention, a fabrication methodof a strained-channel transistor structure where lattice-mismatched zoneextends into drain extension region and/or source extension regionaccording to the present invention is described. A flowchart of thefabrication method in accordance with the fifth embodiment of thepresent invention is shown in FIG. 9. The subsequent descriptions of thefabrication method in accordance with the fifth embodiment of thepresent invention follow the steps in FIG. 9.

In FIG. 10A, a semiconductor substrate such as a silicon substrate 700is provided. Silicon substrate 700 comprises a previously formedplurality of isolation regions (not shown) and previously definedplurality of device regions (not shown). For example, the isolationregions may be shallow trench isolation regions. FIGS. 10A through 10Gprovide a cross-sectional view of one single device region for easydescription. Silicon substrate 700 comprises a channel region 706 in anactive surface. Silicon substrate 700 is N-type doped whenstrained-channel transistor structure 7 in FIG. 10D is a P-channeltransistor structure, or P-type doped when strained-channel transistorstructure 7 in FIG. 10D is an N-channel transistor structure.

In FIG. 10B, first, a gate dielectric layer 712 is formed on the channelregion 706, and a gate electrode 714 is formed on the gate dielectriclayer 712. Finally, spacer 715 is formed on a sidewall of gate electrode714, covering a part of the active surface of substrate 700. Gatedielectric layer 712 is formed by thermal oxidation, thermal oxidationfollowed by nitridation, chemical vapor deposition, physical vapordeposition such as sputtering, or other known techniques to form a gatedielectric layer. Gate dielectric layer 712 is preferably silicondioxide, silicon oxynitride, or combinations thereof with a thicknessbetween about 3 Å to 100 Å, preferably approximately 10 Å or less. Gatedielectric layer 712 may be a high permittivity (high-k) material suchas aluminum oxide (Al2O3), hafnium oxide (HfO₂), zirconium oxide (ZrO2),hafnium oxynitride (HfON), hafnium silicate (HfSiO4), zirconium silicate(ZrSiO4), lanthanum oxide (La2O3), or combinations thereof with anequivalent oxide thickness between about 3 Å to 100 Å. Gate electrode714 is polycrystalline-silicon (poly-Si), poly-crystallinesilicon-germanium (poly-SiGe), refractory metal such as molybdenum ortungsten, compounds such as titanium nitride, combinations thereof orother conductive material(s). Implants known as workfunction implantsmay be introduced in gate electrode 714 to alter the workfunctionthereof. Gate electrode 714 is formed by depositing a gate electrodematerial layer (not shown) over substrate 700, depositing a gate mask(not shown) over the gate electrode material layer, patterning the gatemask to define gate electrode 714, etching the gate electrode materiallayer to form gate electrode 714, and removing the gate mask. Gateelectrode 714 is electrically isolated from channel region 706 by gatedielectric layer 712. Gate dielectric layer 712 is preferably siliconoxynitride and gate electrode 714 is poly-silicon etched using chlorineand bromine chemistry with a high etch selectivity with respect to gatedielectric later 714 in accordance with the fifth embodiment of thepresent invention. Spacer 715 is formed using a deposition and anisotropic etching technique for protecting the sidewall of gateelectrode 714 during a subsequent epitaxy step.

In FIG. 10C, a part or all of the active surface of substrate 700 notcovered by gate dielectric layer 712 and spacer 715 is recessed to format least one recess 709 with a depth d by plasma etching using chlorineand bromine chemistry. The depth d of recess 709 is about 50 Å to 1000Å. An optional anneal may be performed to facilitate silicon migrationto repair etch damage caused by etching recess 709 to smooth the surfaceof recess 709 for a subsequent epitaxy process.

In FIG. 10D, recess 709 is filled with a semiconductor material such assilicon-germanium alloy or silicon-carbon alloy to form alattice-mismatched zone 705 and then a drain extension region 701 andsource extension region 703 are formed oppositely adjacent to thestrained channel region 706′. Thus, strained-channel transistorstructure 7 in accordance with the fifth embodiment of the presentinvention is basically formed. Lattice-mismatched zone 705 is formedusing an epitaxy process such as chemical vapor deposition, ultra-highvacuum chemical vapor deposition, or molecular beam epitaxy. Channelregion 706 is strained to be a strained channel region 706′ whenlattice-mismatched zone 705 is formed. Lattice-mismatched zone 705comprises silicon-germanium alloy where molar fraction of germanium isabout 0.1 to 0.9 when strained-channel transistor structure 7 is aP-channel transistor structure. When strained-channel transistorstructure 7 is an N-channel transistor structure, lattice-mismatchedzone 705 comprises silicon-carbon alloy where molar fraction of carbonis between about 0.01 and 0.04, and may further comprise germanium to besilicon-carbon-germanium alloy where molar fraction of germanium is lessthan ten times that of carbon. A silicon cap 722 is optionally formed onlattice-mismatched zone 705 using an epitaxy process such as chemicalvapor deposition, ultra-high vacuum chemical vapor deposition, ormolecular beam epitaxy. Lattice-mismatched zone 705 and optional siliconcap 722 may be in-situ doped or undoped during the epitaxy process. Whenlattice-mismatched zone 705 and optional silicon cap 722 are undopedduring the epitaxy process, but will be doped subsequently, at whichtime dopants will be activated using a rapid thermal annealing process.Drain extension region 701 and source extension region 703 are formed bydopant diffusion from lattice-mismatched zone 705 and optional siliconcap 722. One or both of the drain extension region 701 and sourceextension region 703 comprise the lattice-mismatched zone 705 andoptional silicon cap 722.

In FIG. 10E, a spacer 716 is formed overlying the spacer 715. Spacer 716is formed using deposition and selectively etching a spacer material(not shown) such as silicon nitride or silicon dioxide. Spacer 716 issilicon nitride in accordance with the fifth embodiment of the presentinvention.

In FIG. 10F, a deeper drain region 702 is formed adjacent to drainextension region 701, and deeper source region 704 is formed adjacent tosource extension region 703. Deeper drain region 702 combines with drainextension region 701, lattice-mismatched zone 705 when available, andoptional silicon cap 722 when available to be a drain region 707, anddeeper source region 704 combines with source extension region 703,lattice-mismatched zone 705 when available, and optional silicon cap 722when available to be a source region 708. Deeper drain region 702 anddeeper source region 704 are formed using ion implantation, PiII, gas orsolid source diffusion, or other known techniques.

In FIG. 10G, a conductive layer 720 is optionally formed on drain region707 and source region 708 to reduce resistances of drain region 707 andsource region 708. Conductive layer 720 is formed using self-alignedsilicide, or other metal deposition. Passivation layers and devicecontacts are subsequently formed so as to finish a device comprisingstrained-channel transistor structure 7 in accordance with the fifthembodiment of the present invention.

Sixth Embodiment

In the sixth embodiment of the present invention, a fabrication methodof a strained-channel transistor structure where lattice-mismatched zoneis formed using an ion implantation process according to the presentinvention is described.

In FIG. 11A, a semiconductor substrate such as a silicon substrate 800is provided. Silicon substrate 800 comprises a previously formedplurality of isolation regions (not shown) and previously definedplurality of device regions (not shown). For example, the isolationregions may be shallow trench isolation regions. FIGS. 11A through 11Dprovide a cross-sectional view of one single device region for easydescription. Silicon substrate 800 comprises a conventional transistorstructure comprising a channel region 806 in an active surface ofsubstrate 800, a gate dielectric layer 812 on channel region 806, a gateelectrode 814 on gate dielectric layer 812, a source region 808 anddrain region 807 oppositely adjacent to channel region 806, and a spacer816 on a sidewall of gate electrode 814, covering a part of the activesurface of substrate 800. Drain region 807 comprises a drain extensionregion 801 and a deeper drain region 802, and source region 808comprises a source extension region 803 and a deeper source region 804.Silicon substrate 800 is N-type doped when strained-channel transistorstructure 8 in FIG. 11C is a P-channel transistor structure, or P-typedoped when strained-channel transistor structure 8 in FIG. 11C is anN-channel transistor structure.

In FIG. 11B, an ion implantation process is performed to introduce ions830 of one or more atomic species with a different atomic size from thatof substrate 800 into drain region 807 and/or source region 808. Gateelectrode 814 and spacer 816 act as a implantation mask during the ionimplantation process. Thickness of spacer 816 is adjustable depending onlattice-mismatched zone 805 in FIG. 11C either extending into drainextension region 801 and/or source extension region 803, or notextending into drain extension region 801 and/or source extension region803, as required.

In FIG. 1C, an annealing step is performed on the substrate 800 to forma lattice-mismatched zone in one or both of drain region 807 and sourceregion 808. Thus, one or both of the drain region 807 and source region808 comprise the lattice-mismatched zone 805. Channel region 806 isstrained to be a strained channel region 806′ when lattice-mismatchedzone 805 is formed. Thus, strained-channel transistor structure 8 inaccordance with the sixth embodiment of the present invention isbasically formed. Lattice-mismatched zone 805 comprisessilicon-germanium alloy where molar fraction of germanium is about 0.1to 0.9 when strained-channel transistor structure 8 is a P-channeltransistor structure. When strained-channel transistor structure 8 is anN-channel transistor structure, lattice-mismatched zone 805 comprisessilicon-carbon alloy where molar fraction of carbon is between about0.01 and 0.04, and may further comprise germanium to besilicon-carbon-germanium alloy where molar fraction of germanium is lessthan ten times that of carbon.

In FIG. 11D, a conductive layer 820 is optionally formed on drain region807 and source region 808 to reduce resistances of drain region 807 andsource region 808. Conductive layer 820 is formed using self-alignedsilicide, or other metal deposition process. Passivation layers anddevice contacts are subsequently formed so as to finish a devicecomprising strained-channel transistor structure 8 in accordance withthe sixth embodiment of the present invention.

Although the present invention has been particularly shown and describedabove with reference to six specific embodiments, it is anticipated thatalterations and modifications thereof will no doubt become apparent tothose skilled in the art. It is therefore intended that the followingclaims be interpreted as covering all such alteration and modificationsas fall within the true spirit and scope of the present invention.

1. A strained-channel transistor structure with improved transportproperties, comprising: a strained channel region comprising a firstsemiconductor material with a first natural lattice constant; a gatedielectric layer overlying the strained channel region; a gate electrodeoverlying the gate dielectric layer; and a source region and drainregion oppositely adjacent to the strained channel region, the sourceregion comprising a source extension region and a deep source region,the drain region comprising a drain extension region and a deep drainregion, one or both of the source region and drain region comprising alattice-mismatched zone comprising a second semiconductor material witha second natural lattice constant different from the first naturallattice constant.
 2. The transistor structure as claimed in claim 1,wherein the lattice-mismatched zone is about 10 to 1000 Å thick.
 3. Thetransistor structure as claimed in claim 1, wherein thelattice-mismatched zone is about 300 to 1000 Å thick.
 4. The transistorstructure as claimed in claim 1, wherein the first semiconductormaterial comprises silicon.
 5. The transistor structure as claimed inclaim 1, wherein the second semiconductor material comprises silicon andgermanium.
 6. The transistor structure as claimed in claim 1, whereinthe second semiconductor material comprises silicon and carbon.
 7. Thetransistor structure as claimed in claim 5, wherein molar fraction ofgermanium in the second semiconductor material is about 0.1 to 0.9. 8.The transistor structure as claimed in claim 6, wherein molar fractionof carbon in the second semiconductor material is about 0.01 to 0.04. 9.The transistor structure as claimed in claim 6, wherein the secondsemiconductor material further comprises germanium whose molar fractionin the second semiconductor material is less than ten times that ofcarbon in the second semiconductor material.
 10. The transistorstructure as claimed in claim 1, wherein the strained channel region isunder a tensile strain in a source-to-drain direction.
 11. Thetransistor structure as claimed in claim 10, wherein the tensile strainis about 0.1% to 4%.
 12. The transistor structure as claimed in claim 1,wherein the strained channel region is under a compressive strain in avertical direction.
 13. The transistor structure as claimed in claim 11,wherein the compressive strain is about 0.1% to 4%.
 14. The transistorstructure as claimed in claim 1, wherein the strained channel region isunder a compressive strain in a source-to-drain direction.
 15. Thetransistor structure as claimed in claim 14, wherein the compressivestrain is about 0.1% to 4%.
 16. The transistor structure as claimed inclaim 1, wherein the strained channel region is under a tensile strainin a vertical direction.
 17. The transistor structure as claimed inclaim 16, wherein the tensile strain is about 0.1% to 4%.
 18. Thetransistor structure as claimed in claim 1, further comprising a caplayer on the lattice-mismatched zone.
 19. The transistor structure asclaimed in claim 18, wherein the cap layer comprises the firstsemiconductor material.
 20. The transistor structure as claimed in claim1, wherein the lattice-mismatched zone laterally extends into the sourceextension region and/or drain extension region.
 21. The transistorstructure as claimed in claim 1, wherein the gate dielectric layer issilicon oxide or silicon oxynitride.
 22. The transistor structure asclaimed in claim 1, wherein the gate dielectric layer is Al2O3, HfO2,ZrO2, HfON, HfSiO4, ZrSiO4, La2O3, or combinations thereof.
 23. Thetransistor structure as claimed in claim 22, wherein the relativepermittivity of the gate dielectric layer is larger than
 5. 24. Thetransistor structure as claimed in claim 1, wherein the gate dielectriclayer is about 3 to 100 Å thick.
 25. The transistor structure as claimedin claim 1, wherein the gate electrode comprises poly-crystallinesilicon.
 26. The transistor structure as claimed in claim 1, wherein thegate electrode comprises poly-crystalline silicon germanium.
 27. Thetransistor structure as claimed in claim 1, wherein the gate electrodecomprises a metal.
 28. The transistor structure as claimed in claim 1,wherein the gate electrode comprises a metal silicide.
 29. Thetransistor structure as claimed in claim 1, further comprising aconductive material on the source region and drain region.
 30. Thetransistor structure as claimed in claim 29, wherein the conductivematerial comprises a metal.
 31. The transistor structure as claimed inclaim 29, wherein the conductive material comprises a metal silicide.32. A strained-channel transistor structure with improved transportproperties, comprising: a strained channel region comprising a firstsemiconductor material with a first natural lattice constant; a gatedielectric layer overlying the strained channel region; a gate electrodeoverlying the gate dielectric layer; and a source region and drainregion oppositely adjacent to the strained channel region, one or bothof the source region and drain region comprising a lattice-mismatchedzone comprising a second semiconductor material with a second naturallattice constant different from the first natural lattice constant,wherein the lattice-mismatched zone further extends beyond the sourceregion and/or drain region.